1. Field of the Invention
The present invention relates to a dipole antenna and radio-frequency device, and more particularly, to a dipole antenna and radio-frequency device having a balun to balance a feed-in impedance.
2. Description of the Prior Art
An antenna is used for transmitting or receiving radio waves, to communicate or exchange wireless signals. An electronic product with a wireless communication function, such as a tablet computer, a laptop or a personal digital assistant (PDA), usually accesses a wireless network through a built-in antenna.
Please refer to FIG. 1, which is a schematic diagram of an RF (Radio-Frequency) device 10. The RF device 10 has a function of wireless communication; take a note book computer for example. The RF device 10 includes an antenna 11, an RF signal process unit 12 and a housing 13. In general, to prevent the antenna 11 from being disposed within a metallic environment, such as a central area disposed with metal parts, a hard disk, input-output ports or a mother board (not shown in FIG. 1), the antenna 11 is normally disposed on a border of the housing 13. Thus, it is usual to use a metal wire, e.g. a co-axial cable 14, to transmit an RF signal received and radiated by the antenna 11 to the RF signal process unit 12 for further signal process.
However, the above mentioned design principle may cause the co-axial cable 14 for transmitting the RF signal to become a part of a radiator of the antenna 11. If the co-axial cable 14 is interfered by noises, the RF signal will be interfered by noises as well, and a signal quality of the RF signal may be decreased accordingly.
On the other hand, the co-axial cable 14 may have different levels of influence on antenna performances according to different antenna types. For example, a gain of a dipole antenna is theoretically higher than a gain of a monopole antenna and also higher than a gain of a PIFA (Planar Inverted-F Antenna), but the co-axial cable 14 may unbalance a feed-in impedance of the dipole antenna. As a result, the antenna performance of the dipole antenna may be changed once the co-axial cable 14 is changed, e.g. impedance changes by cable routes, which may decrease stability and reliability of the dipole antenna 11 during manufacture.
Therefore, how to design the dipole antenna having a stable performance and a balanced feed-in impedance to improve the stability and the reliability during manufacture has become a topic in the industry.